1. Field of the Invention
The present invention relates to a branching filter (duplexer) using a surface acoustic wave (SAW) filter, and more particularly, to a branching filter which provides harmonic suppression and high surge resistance without increasing the number of components and the size of the device, and also relates to a communication apparatus including the duplexer.
2. Description of the Related Art
With the recent popularity of mobile communication devices, such as cellular phones, the demand for thinner and smaller cellular phones with stable operation, e.g., with fewer malfunctions over time, has increased.
Such cellular phones share an antenna between the transmitter and the receiver in order to reduce the size. The cellular phones include a branching filter (hereinafter referred to as a duplexer or DPX) for separating a transmission high-frequency signal and a reception high-frequency signal whose center frequency is different from that of the transmission high-frequency signal. The duplexer includes a transmitting filter and a receiving filter which are connected to the antenna.
FIG. 11 shows a duplexer 50. As described in, for example, Japanese Unexamined Patent Application Publication No. 6-350307, the duplexer 50 includes a low-pass filter between, for example, an antenna (ANT) and a receiving (Rx) filter 52, defining a matching circuit for suppressing mutual interference between the transmitter (Tx) side and the receiver (Rx) side. The low-pass filter has a three-section structure including parallel capacitors 57 and 59 and a series inductor 58.
The matching circuit of the duplexer 50 makes the impedance of the Rx filter 52 infinite at the frequencies of the other filter for the antenna, i.e., the passband frequencies of a transmitting (Tx) filter 51. The matching circuit also minimizes change in the impedance of one of the filters at the passband frequencies of the other filter when the filters are connected to, thus, suppress mutual interference.
The duplexer is also required for suppressing harmonics, particularly, second and third harmonics. FIG. 12 is a circuit block diagram of a typical cellular phone. In the cellular phone, a power amp (PA) 61 is connected to an input terminal of a Tx filter 51 of a duplexer 50, and communication signal harmonics are caused by the characteristic of the PA 61. Such harmonics are also emitted from the antenna which thus causes noise. In order to overcome this problem, in the related art, an isolator is connected between the input terminal of the Tx filter 51 and the PA 61 at a location indicated by a dotted circle 63.
In order to support high-passband filters for use in cellular phones, use of compact surface acoustic wave filters (hereinafter referred to as SAW filters) has been considered. In the SAW filters, a plurality of interdigital transducers (hereinafter referred to as IDTs) having a plurality of interdigitated electrode fingers are provided on a piezoelectric substrate in the SAW propagation direction. Preferably, the SAW filters further include reflectors on both (right and left) sides of each IDT in the SAW propagation direction.
However, duplexers using a SAW filter also have a problem in that harmonics (second, third, . . . , nth harmonics) at the passband frequencies pass through the SAW filter because of its structure. The harmonics input from the PA 61 also passes through the SAW filter which leads to noise.
Another problem is that the electrode pitch of the SAW filter is very narrow, such as submicron to 2 μm, which results in low surge resistance. Incoming static electricity from the antenna during cellular phone use can damage the SAW filter, and ESD (electrostatic discharge) protection is therefore required.
When the above-described duplexer shown in Japanese Unexamined Patent Application Publication No. 6-350307 uses SAW filters as the Tx filter 51 and the Rx filter 52 in order to reduce the duplexer size, an additional isolator is required between the Tx filter 51 and the PA 61 to suppress a harmonic input from the PA 61 to the Tx filter 51, or an additional isolator is required between the antenna and the Tx filter 51 to suppress a harmonic in this SAW filter. If the duplexer includes an additional protection circuit for improving the surge resistance of the SAW filters, the number of components, such as the isolator and the protection circuit, increases and the duplexer size inevitably increases.
Japanese Unexamined Patent Application Publication No. 7-226607 discloses a duplexer using a SAW filter in which a strip line (matching circuit) provided in an internal layer of a multi-layer dielectric substrate is connected to a SAW filter provided on the principal surface of the substrate via a via hole. The duplexer disclosed in this publication does not provide ESD protection.
Japanese Unexamined Patent Application Publication No. 2001-352271 discloses an electrostatic protection circuit which is connected to an antenna of a mobile communication apparatus. In this publication, a parallel-connected transmission line permits incoming static electricity from an antenna to escape to a ground, which prevents failure caused by electrostatic discharge. However, this electrostatic protection circuit does not provide harmonic suppression, and thus, requires an additional trap circuit for harmonic suppression.
Japanese Unexamined Patent Application Publication No. 2001-127663 discloses that a circuit which is defined by a parallel inductor and a series capacitor and which is connected between an antenna and a filter is added to a high-frequency switching module for electrostatic protection. In this publication, a multi-layer substrate is used in order to prevent an increase in the size of the circuit because of the space required for the additional circuit. In this structure, a trap circuit for harmonic suppression and a circuit for electrostatic protection are separately required, thus increasing the number of components.